The power factor of a system is defined as real input power divided by apparent input power. Apparent power is calculated by multiplying root mean square (RMS) voltage and RMS current. Therefore, a system with a lower power factor requires larger input currents for a given real power than a system with a higher power factor.
Electronic circuits, such as gas discharge lamp ballasts, generally operate from DC voltages. Hence, a means for converting AC line voltage to DC line voltage is required. A conventional way to obtain a DC voltage from an alternating voltage source is through rectification and capacitive filtering. However, in a circuit used to accomplish this result, the power factor, which depends upon the value of the filter capacitor and the size of the load, is often too low. Therefore, the input current necessary to support input power is very high and can exceed the ratings of conductors and circuit breakers.
Reducing the filter capacitor value improves the power factor, but increases the ripple on the output voltage. This is undesirable because gas discharge lamp ballasts have a further requirement that the ripple on the output voltage not exceed a certain value. This ripple specification, known as the crest factor, is obtained by dividing the peak output voltage by the RMS output voltage. The crest factor is required to be less than 1.6 for fluorescent lamps, for example, because a higher crest factor is known to adversely affect the operating life of the lamp. As a result, the requirements of higher power factor and lower crest factor are in conflict.
The lighting industry has long recognized the advantages of high power factor circuits, and thus a power factor greater than 0.9 has become a de facto requirement of ballasts for gas discharge lamps. Conventional, or electromagnetic, ballasts for gas discharge lamps include bulky low-frequency transformers and inductors and large power factor correction capacitors. The inductors and transformers are necessary to limit the current in the lamps once they are ignited. This is due to the negative impedance characteristic of gas discharge lamps; that is, once the arc of a discharge lamp has been ignited, the current through the discharge medium increases, while the voltage drop between the lamp electrodes decreases. Therefore, an electromagnetic ballast acts as an inductive load, and a large power factor correction capacitor is used to increase the power factor.
In contrast to conventional electromagnetic ballasts, the load presented by electronic ballasts to the AC line is capacitive. This is due, in part, to the above-described filter capacitors. In addition, smaller transformers and inductors are used because electronic ballasts operate at higher frequencies, i.e. 20-50 kHz. Thus, a power factor correction circuit is needed to correct for a capacitive load at high frequency operation.